If there is still some of the shank exposed after taking the whole manifold off you might be able to get enough of a hold on it with a pair of locking jaw pliers and undo them. If you don't replace them the worst thing that will happen is an exhaust leak from the manifold. It will run like a pig but it'll run. An as for replacing them just go to a junk yard and pilfer a few off any engine with the same pitch thread (they're metric so pick something out of the same family - DSM, Hyundai, Kia, Mitsubishi etc)